The ability to remotely activate a switch is a desirable property of communication networks. For example, electronic mail is a rapid and efficient method of communication between computer users. When an electronic mail message is successfully sent to a computer user, the message is stored in a file by the host computer at the receiving location (or, more generally, by a host computer in the receiving local area network) in association with the receiving user's address. If the receiving user is logged on the host computer, the host computer can readily send mail to the user or, alternatively, a message to the user indicating that he has mail waiting. However, if the receiving user's terminal is turned off, or logged on to a different host computer, the user has no way of knowing that he has mail waiting. As a consequence, electronic mail can be an effective method of mail delivery only if the user frequently logs on to a host computer. A potential solution to this problem is to have a "mail waiting" alarm that can be remotely activated even when the user's terminal is turned off or logged on to a different host computer.
Various approaches have been made to solving this and related problems, such as the problem of sending "mail waiting" or "message waiting" signals to individuals, such as hotel guests, who are served by a telephone switching system. Some approaches, as described, for example, in U.S. Pat. No. 4,582,959, issued to Myslinski, et al. on Apr. 15, 1986, store information about a received message in a database at the receiving location. Computer software residing in a computer at the receiving location checks the database and activates a signal light at the user's station or terminal to alert the user to the status of his recent messages. An alternative approach, described, for example, in U.S. Pat. No. 4,072,825, issued to McLay, et al. on Feb. 7, 1978, makes use of a microprocessor, under hardwire program or firmware command, at the receiving location to perform similar functions.
Both of these approaches require a data-processing unit associated with the message-receiving device to be operating at least part of the time between receipt of the message and successful notification of the user for whom the message is intended. Such approaches are disadvantageous because they make use of relatively expensive computational power to identify the status of messages and to signal the user. Moreover, such approaches require the relatively expensive computational power to be available either continuously, or at frequent intervals.
Some other approaches, as described, for example, in U.S. Pat. No. 4,086,441, issued to Ullakko, et al. on Apr. 25, 1978, make use of individual signaling paths, consisting of electric conductors, in order to send the activating signal. Still other approaches require a human operator to activate the "mail waiting" signal by keying in the address of the receiving station.
For reasons of economy, it is advantageous for the activating signal to be routed to the receiving station automatically (i.e., without human intervention). For example, a simple way to achieve this for the message itself to contain, or generate, the activating signal. Also for reasons of economy, it is advantageous for the activating signal to be transmissible without the need for a special signaling path. As noted above, however, even those approaches that involve automatic transmission of the activating signal through an ordinary data transmission medium still must rely upon the continuous or frequent availability of a relatively expensive data-processing unit at the receiving end.
Thus, those in the communication systems art have long sought, so far without success, a digital communication system in which a remote switch, located near a receiver and connected to the same communication medium as the receiver, can be activated by a signal routed to the receiver, without the mediation of a data-processing unit, even when the receiver is powered off.